Landracing Forum
Tech Information => Technical Discussion => Topic started by: StraightSix on May 10, 2005, 02:00:00 PM
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Assuming you have a locking torque convertor, is there any disadvantage to running an automatic transmission, with regard to drivetrain power losses?
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Yes
JB
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Okay.
(Roughly how much %?)
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What automatic are you considering?
Sum
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Well all the automatics are somewhat different in reguards to power loss, but as an example a T-400 eats up about 50 HP.
JB
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Toyota A340E.
So the power loss would be more of a set figure and not so much a percentage of the power transmitted?
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I got the following gear ratios from another site:
Gear ratios for the A340E 92 SC400 US model are
1st 2.804
2nd 1.531
3rd 1.0
4th 0.705
Rev 2.393
That is actually a pretty wide ratio transmission with a strong overdrive gear. Think how hard it is to pull overdrive on a big hill. According to Costella, and I agree, "Bonneville can be like pulling a big hill". So if you gear the car higher in the rear than you would on the highway what is going to happen when it shifts into 4th. If it is like my truck it will fall on it's face.
Some of the 5/6 speed manual overdrives only have about a 20% drop going into 5th and/or 6th. Much better. I see that GM/Chrysler have developed a 6 speed automatic jointly that will be coming out in future models.
Still like mentioned above it is going to take more HP to turn the auto vs. the manual, so why give that HP up especially if you are pursuing a record.
I guess is there a reason that for you the automatic is desirable?
c ya, Sum
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The only real reason is that that's what the car comes with. I agree the ratios are rather wide and a peaky powerband could present a serious problem.
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Has anyone ever done any real testing of transmissions and or rearends on a chassis dyno? I have never seen one.
I think it would be very interesting to run a T10 - lenco - Liberty - Richman - Jerico - TH400 - TH350 - C4 - etc back to back on a chassis dyno to see the real numbers.
Maybe I'm just skeptical because for years I was told so many things about air flow that when I got a flow bench I found most of them were not true.
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Forgot to mention OD. Torque division...unless you got a serious blower motor it's gonna be tough to pull a .7 OD gear regardless of whether is an automatic or manual.
JB
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Mike Powell from Tex Racing has done a bunch of testing, but mostly on the Super T-10, T101 and Jerico 4-speeds.
JB
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And results of these test are..........???????
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If an automatic transmission had 50 lb of steel , 20 lb of aluminum and 15 lb of oil and if it isn't water cooled every degree it heats up is 75 BTUs . 75 BTUs in a 1 1/2 minute run is an average of 1.2 HP (1.2 x degrees gained = HP) .
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For actual numbers someone will have to ask Mike. I'll see what I can pry out of him the next time we talk. I can tell you that, acording to Mike, the typical NASCAR 4-speed is the most effiecient and least HP hungry when compared to any automatic and most 5-speeds.
JB
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no torque converter== very little heating
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I think that you need to look at the way the trannys are built. As Sparky says if you get rid of the torque converter you really help an automatic. An automatic is usually two sets of planetary gears, one in the case of a Powerglide, plus they need an oil pump to make things work. Planetary gear sets are typically 95 to 97% efficient, so in series that would make them 90 to 94% total plus the oil pump drive and the internal "windage" so it would be easy to have an automatic that was less than 90% over all efficiency. Maybe that's why transmission coolers are so popular with automatics.
Standard trannys, once they get into high gear, and if the tranny is not an over drive type, then the power is transferred from the main drive gear straight through the top shaft to the tail shaft and there are no gear meshes that are involved in transmitting the power. Whereas an over drive tranny requires that the power goes from the main drive input gear to the cluster(one gear mesh) and then from the cluster (counter shaft as some call it) through the over drive gear (a second gear mesh) and out the tail shaft. Each gear mesh is probably a 2-3% power loss. So I would vote for a manual trans without an overdrive top gear as the most efficient. There is also hp to be found in using thinner transmission fluid in a standard tranny but you need to dry sump it and do some oil spraying on the gears, but you can easily gain more than it takes to drive the oil pump.
Rex
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I am with the Contrivance Engineering group gmr-e. We have been running a th350 with a solid hub replacing the converter. in 2005 it got very hot and metal was found in the pan. We do have a car mounted dyno(for a plug, go to contrivanceengineering.com to see the dyno) and we are getting around 100hp loss at speed. Thats alot, but we must also remember it could have been hurt during these readings...Hans